By definition, the frame-rate of a video sequence is constituted by the temporal frequency of said sequence, i.e., by the number of digital images, organized in frames, per second that form the sequence itself.
FIG. 1 shows a block diagram representing a system for frame-rate conversion, which comprises a source video system 100, which produces at output a video sequence SV of images with variable frame-rate v, and a destination video system 120, which is instead configured for receiving a video sequence SC at constant and pre-set frame-rate c. Consequently, in order to enable transmission of video sequences from the source video system 100 to the terminal video system 120, there is provided a frame-rate converter apparatus 110, which receives at input the video sequence SV with variable frame-rate v and generates at output the conversion into a video sequence SC with constant and pre-set frame-rate c.
In a typical scenario of application, the source video system 100 corresponds to a video decoder, which operates, for example, but not only, according to a standard selected from among video standards such as MPEG-2, MPEG-4, H.264, DivX, and Windows Media Video. In the case where the source video system 100 is, for example, represented by a decoder, it receives a bitstream, compressed through a channel, whether wired or wireless, which may introduce errors and/or variable delay or jitter, so that, according to the delay introduced by the channel itself, the source video system 100 is able to decode the images contained in the bitstream at a higher rate or at a lower rate. The terminal video system 120 can be, for example, a viewing device, i.e., a display, with a pre-set refresh frequency, capable of receiving and displaying a pre-set number of images per second corresponding to the pre-set constant frame-rate c.
The frame-rate converter apparatus can operate according to the known simple method of conversion from a variable frame-rate v to a constant frame-rate c, which envisages the following conditions:                when the variable frame-rate v is higher than the constant frame-rate c, it is envisaged to eliminate the images in excess (frame-skipping operation) avoiding display thereof; and        when the variable frame-rate v is lower than said constant frame-rate c, it is envisaged to repeat the input images (frame-repetition operation) a number of times.        
It is clear how both the operation of frame skipping and that of frame repetition may cause evident and troublesome temporal display artifacts.
Known from U.S. Pat. No. 5,796,439, which is incorporated by reference, is, for example, a method for reduction of the frame-rate of a video sequence by means of selective frame skipping.
European patent application No. EP-A-0 637 889, which is incorporated by reference, describes a method for varying the frame-rate of a video sequence by means of motion-compensated interpolation. However, the method is not dynamic and has only the purpose of doubling the frame-rate of the video sequence to be displayed. A similar method for frame-rate doubling is also known from European Patent Application No. EP-A-0 475 499, which is also incorporated by reference.
The U.S. Patent Application Publication No. US2004/156624, which is incorporated by reference, describes a system capable of dynamic adaptation of the rate of display of a digital video sequence via a buffering operation, i.e., an operation of temporary storage, and subsequent check on the buffer. However, said system does not render the output frame-rate constant.